Mixture Ratio Feedback Control System

MFI System Operation/Closed Loop:

PURPOSE
The mixture ratio feedback system is used for precise control of the mixture ratio to the stoichiometric point, so that the three way catalyst can reduce Co, HC and NOx emissions.

OPERATION
This system uses a heated oxygen sensor in the exhaust manifold to check the air-fuel ratio. The ECCS Control Module adjusts the injection pulse width according to the sensor voltage so the mixture ratio will be within the range of the stoichiometric air-fuel ratio. This stage refers to the closed loop control condition.

OPEN LOOP CONTROL
The open loop control condition refers to that under which the ECCS Control Module detects any to the following conditions and feedback control stops in order to maintain stabilized fuel combustion.
^ Deceleration
^ High-load, high-speed operation
^ Engine idling
^ Malfunction of heated oxygen sensor or its circuit
^ Insufficient activation of heated oxygen sensor at low engine coolant temperature
^ Engine starting

MIXTURE RATIO SELF-LEARNING CONTROL
The mixture ratio feedback control system monitors the mixture ratio signal transmitted from the heated oxygen sensor. This feedback signal is then sent to the ECCS Control Module to control the amount of fuel injection to provide a basic mixture ratio as close to the theoretical mixture ratio as possible. However, the basic mixture ratio is not necessarily controlled as originally designed. This is due to manufacturing differences (e.g., mass air flow sensor hot film) and changes to the Electronic Concentrated Engine Control System parts during operation (injector clogging, ect.) which directly affect the mixture ratio.
Accordingly, the difference between the basic and theoretical mixture ratios is monitored in this system. This is then computed in terms of "fuel injection duration" to automatically compensate for the difference between the two ratios.